[web] inform password users that they need to log in from native to claim username

Summary:
Depends on D12545

this is similar to the previous diff but for password users

Test Plan:
tried logging in from web with a reserved username and got the expected error message

Reviewers: inka, ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12546